(1. ) Field of the Invention
The present invention relates to a method for sequential control of the operation of an apparatus for modifying the surface of polymer products with microwave-excited oxygen plasma.
(2. ) Description of the Prior Art
As is well known, a recent trend in the automotive industry has been to make automobile parts from plastic materials. Plastic materials offer reduced bulk density and their moldability affords wider designing flexibility.
The relatively inexpensive and readily available polyolefins such as polypropylene and polyethylene, however, are difficult to paint. Further, coatings on the surfaces of polyolefin products tend to peel easily.
To improve the paintability, wettability, and bondability of polyolefins, methods are known in the art for modifying the surface of polyolefins. In such methods, the polyolefin surface is subjected to oxygen plasma or activated oxygen gas species generated by glow discharge, corona discharge, radio frequency discharge, or microwave discharge, for the dual purposes of plasma oxidizing the surface to introduce polar radicals therein and of etching the surface to increase mechanical anchoring. For example, R. M. Mantell et al. have reported that low temperature oxygen plasma improves the surface-bonding characteristics and wettability of polyolefins (Ind. Eng. Chem. Prod. Rev., Vol. 3, No. 4, December 1964, pp. 300-303). Also, J. R. Hall et al. have reported that the bondability of polypropylene and polyethylene is increased by oxygen plasma surface treatment (J. Appl. Polym. Sci., Vol. 13, 1969, pp. 2085-2096).
The results of these experiments triggered the development of plasma treatment apparatuses capable of modifying polymer surfaces on a commercial and industrial scale. For example, Japanese Unexamined Patent Publication (Kokai) No. 53-85782 (1978) discloses an apparatus for modifying a polymer surface with microwave-excited oxygen plasma which comprises a microwave power generator connected by a waveguide with a microwave discharge tube to which a stream of oxygen gas is supplied from a gas source. The oxygen gas in the discharge tube is excited by microwave discharge into oxygen plasma which is introduced into a reactor to contact and surface modify the polymer articles received therein. The reactor is a sealed enclosure defining a reaction chamber and has a movable door to permit access to the reaction chamber. The reactor is connected to a vacuum pump and is held on a substantially vacuum condition during plasma treatment. In operation, the access door is opened and a desired number of polymer articles to be surface treated are placed in the reactor. Following closure of the door, the vacuum pump is operated to evacuate the reaction chamber, and the oxygen gas, together with other reaction gases as required, is allowed to flow through the microwave discharge tube into the reactor. As the pressure within the reactor decreases down to a required level, i.e., as the reaction chamber is subjected to a vacuum of a certain level, the microwave power generator is energized to generate oxygen plasma. After treatment, the access door is opened and the treated articles are taken out. Thus, the plasma processings according to this apparatus are performed on "batch" cycles.
The result or degree of plasma surface treatment and, hence, the surface quality of the treated articles, depends on processing factors such as the pressure within the reactor, the flow rate of oxygen gas, and the input power of microwave discharge. Conventional methods of testing and analyzing the results of plasma surface treatment include X-ray electron spectroscopy for chemical analysis (ESCA) and Fourier transform infrared spectroscopy (FF-IR). The simplest method may include the measurement of the contact angle, whereby the wettability of treated surface is quantitatively measured. All of these testing methods are based on the measurement of the quantity of hydrophilic radicals such as --OH, &gt;C.dbd.O, and --NHC.dbd.O formed in the outermost layer of the treated articles through a thickness of from 10 to about 500 angstroms. Although these methods are effective for testing and analysis of the results of plasma treatment, they nevertheless are not applicable to commercial plasma treatment apparatuses for controlling the surface quality of articles under processing because the treatment is conducted in batch cycles, as mentioned above, and since the testing and measurement can be conducted only after completion of plasma processing.